**Publication date:** May 2010

**Abstract:**

Boutloukos et al. discovered twin-peak quasi-periodic oscillations

(QPOs) in 11 observations of the peculiar Z-source Circinus X-1. Among

several other conjunctions the authors briefly discussed the related

estimate of the compact object mass following from the geodesic

relativistic precession model for kHz QPOs. Neglecting the neutron star

rotation they reported the inferred mass M 0 = 2.2 ±

0.3 M sun. We present a more detailed analysis of the

estimate which involves the frame-dragging effects associated with

rotating spacetimes. For a free mass we find acceptable fits of the

model to data for (any) small dimensionless compact object angular

momentum j = cJ/GM 2. Moreover, quality of the fit tends to

increase very gently with rising j. Good fits are reached when M ~ M

0[1 + 0.55(j + j 2)]. It is therefore impossible

to estimate the mass without independent knowledge of the angular

momentum and vice versa. Considering j up to 0.3 the range of the

feasible values of mass extends up to 3 M sun. We suggest

that similar increase of estimated mass due to rotational effects can be

relevant for several other sources.

**Authors:**

Török, Gabriel; Bakala, Pavel; Šrámková, Eva; Stuchlík, Zdeněk; Urbanec, Martin;